1. Field of the Invention
The present invention relates to a glow plug having a ceramic heater held in a cylindrical sleeve press-fitted in a cylindrical housing, and a method of producing such a glow plug.
2. Description of the Related Art
Glow plugs, having a rod-like ceramic heater capable of generating heat when energized and a cylindrical sleeve press-fitted in a cylindrical housing with the ceramic heater held therein, are known heretofore as disclosed, for example, in Japanese Patent Publication (JP-B) No. 3-13485.
FIG. 15 hereof shows in longitudinal cross-section a part of a typical example of such known glow plugs.
As shown in FIG. 15, the known glow plug includes a cylindrical housing 110 in which a cylindrical sleeve 120 is press-fitted. The sleeve 120 has one end portion projecting from an end 111 of the housing 110 and the other end portion press-fitted in the housing 110. The other end portion of the sleeve 120 thus forms a press-fit portion 123 where the sleeve 120 and the housing 110 are secured together.
A rod-like ceramic heat generating member or heater 133 that can generate heat when energized or supplied with electricity is held in the sleeve 120 with one end portion 131 projecting from a projecting end 121 of the sleeve 120 and the other end portion thereof inserted in the sleeve 120.
The heater 130 includes a heater element 133 of electrically conductive ceramic buried in a ceramic insulator 135. Two lead elements 134 are connected to opposite ends of the heater element 133. One of the lead elements 134 has an end portion exposed to an outer peripheral surface of a portion of the insulator 135 that is located inside the sleeve 120.
The thus exposed end portion of the lead element 134 and an inside surface of the sleeve 120 are joined together by brazing for electric connection and form a joint portion 136 where the heater 130 and the sleeve 120 are electrically connected with each other.
In the manufacture of the glow plug, the rod-like ceramic heater 130 is inserted in the sleeve 120 in such a manner as described above, and the sleeve 120 is then press-fitted in the housing 110 in such a manner as described above with the heater 130 held in the sleeve 120.
In the known glow plug of the foregoing construction, the joint portion 136 is very weak or brittle at a heater side thereof because of the presence of a metal wire or a ceramic material of a different kind from the ceramic insulator 135 is buried as the lead element 134 in the ceramic insulator 135.
When the sleeve 120 is secured by press-fitting to the housing 110 with the heater 130 held therein, the joint portion 136 between the sleeve 120 and the heater 130 is subjected to a load or pressure (hereinafter referred to as “press-fitting load”) as it overlaps with the press-fit portion 123.
Application of the press-fitting load to the joint portion 136 which is weak or brittle at the hater side thereof may result in the generation of micro-cracks, which will cause a conduction failure between the lead element 134 and the sleeve 120 or lower the operational reliability, in performing repeated energization of the heater element 133.
The sleeve 120 and the housing 110 may be secured together by brazing rather than by press fitting. In the brazing process, only a small load is applied to the joint portion 36 and, hence, the heater 130 has little fear of rupturing at the joint portion 136. The brazing process is, however, time-consuming, cost-ineffective as compared to the press-fitting process.